Much easier to access to rummage around for stuff is the main one. Easier to attach to bike too probably (no rubbing etc), don't need to pack carefully, work fine when empty, ready access via side pockets.
Imo the sausage seatpacks are only better at being lightweight and more stable over sketchy terrain. If you're mostly on roads a carradice will be fine.
Or just do what most people on here do (self included) and buy whichever will look cooler on your frame.